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Tuesday, February 19, 2013

Brother, Can You Spare $1B for a Planetary Space Telescope?

Imagine you had a Hubble-class telescope and could use in any way you
wanted to explore planets. What would
you do with it?

A number of scientists have had the chance to explore that question
courtesy of an American spy agency. A
year ago, NASA received a surprise phone call from the National Reconnaissance
Agency (NRO), which flies spy satellites, asking if it would like two spare,
Hubble-class space telescopes. NASA’s
managers said,”Yes!,” and now the agency is looking for the best uses for the
telescopes. The highest priority is to
see if one telescope could be used to meet the needs for the Wide-Field
Infrared Survey Telescope (WFIRST) to study dark energy. That potentially leaves a second telescope
for other deep space or solar system studies.

Before I describe the concepts discussed at a recent conference, a
truth in advertising statement is required.
Each telescope represents $250M worth of hardware. However, not all the systems needed for an
actual satellite are included, instruments are not included, and the whole lot
would need to be launched. Figures I’ve
read suggest that turning each telescope into a working observatory would cost
approximately $1B, depending on the specifics of the mission. NASA’s current budgets have no room to fund a
single mission, much less two. In a few
years, there may be room for a mission or two, so the space agency is
soliciting ideas.

Some assembly required: The hardware provided by NRO. NASA would be required to provide all the additional hardware, launch, and mission operations support to turn the core telescope into a working space telescope. Image from this presentation.

At the conference, researchers presented ideas for both deep space
astronomical targets and a variety of planetary targets. The latter were divided between exploring
Mars, monitoring the solar system, and examining any planets orbiting stars in
the stellar neighborhood.

The original mission for the telescopes was to take very high
resolution images of the Earth’s surface.
Two of the proposals would repurpose those terrestrial imaging
capabilities to spy the surface of the Red Planet in very high resolution. The Mars Reconnaissance Orbiter’s instruments
are the current high resolution champs at Mars with 90 cm resolution for images
and 18 m for multi-spectral imaging. By
using the NRO’s telescope, those numbers improve to 23 cm and 1 m. A second proposal would improve those numbers
to 8 cm and 21 cm. (Those improvements
appear to also come in part from more modern instruments as well as the larger
telescope.) The high resolution comes
with a tradeoff, and only a handful of sites could be studied each day. Depending on the resolution, the images taken
would be stripes along the surface less than two or even one kilometer wide.

Getting this large of a telescope to Mars, however, would be a
challenge. Both proposals would use a
solar electric propulsion stage. While
smaller planetary spacecraft have used solar electric propulsion, I believe
this would be the first with such a large spacecraft. One of the proposals points out that this
would be a demonstration of solar electric propulsion capabilities that may
eventually be used for a human Mars mission.

Comparison of potential capabilities of the NRO telescopes in Mars orbit for visible camera images and for an imaging spectrometer compared to the capabilities of the currently operating Mars Reconnaissance Orbiters HiRISE camera and CRISM imaging spectrometer. While the NRO telescope would improve resolution, the size of the images would be much smaller (look at the swath widths). From Zachary J. Bailey and colleagues abstract submitted to the conference. Click on the table for a larger image.

Several of the proposals would keep the telescope near Earth, but would
use it to watch the bodies of the solar system.
The studies proposed are not that different than what can be done with
Hubble today. However, the Hubble
telescope spends just a small portion of its time looking at the
neighborhood. As one proposal summary
states: “The advanced instruments on [Hubble] are rarely trained on solar
system targets due to high oversubscription and an increasing focus on deep
field observations.”

The solar system watch proposals presented at the conference point out
that the instruments proposed would also be useful for deep space studies, the
allocation of observing time would be reversed.
The telescope’s resolution and would allow it to be a planetary mission
to many worlds. Many of the studies
would repeat observations over months and years: the change in weather patterns
on the gas planets as well as Saturn’s moon Titan; and the evolution of
outgassing by comets as they approach and recede from the inner solar system; and
volcanism on Io and Enceladus (with searches for volcanism on other
worlds). Other observations would study
large numbers of asteroids and Kuiper belt objects in the distant solar system
to measure their size and composition.
Depending on the specific proposal, the instrument complement would
include some combination of a camera for high resolution images and/or
ultraviolet and near infrared spectrometers for composition measurements.

A number of the proposals would use an NRO telescope to perform long term solar system studies. This image shows Uranus as imaged by the Hubble Space Telecope.

A number of the proposers would look beyond our solar system to study planets
orbiting stars in our stellar neighborhood up to 60 light years away. While the specifics of each proposal varied,
most proposed would use one or more of three techniques:

A coronagraph could block the bright light of
the target star, allowing planets to be visible for spectrographic studies of
their composition.If the coronagraph is
built into the telescope, it appears that gas giants and possibly super
Earth-sized planets could be studied.If
a co-orbiting stellar shade were used instead, Earth-sized worlds within inner
stellar systems could be detected.

A second instrument could use astrometric
techniques to measure extremely faint wobbles in the star’s motion to determine
the mass of visible planets and to detect smaller planets too small to
image.While similar techniques have
been used to discover numerous exo-planets with Earth-bound telescopes, a large
telescope above Earth’s atmosphere would allow much more sensitive searchers

A third instrument could measure the spectral
change that occurs when planets cross in front of their stars.The bright light of the star shining through
the atmosphere enables more precise compositional measurements for planets with
atmospheres than is possible with a coronagraphic spectrometer, which can
examine only the faint reflected light.For all planets that happen to cross in front of the star, the dimming
of the light received provides information about the size of the planet.

We know so little about planets orbiting other stars that even simple measurements of colors can tell us what type of world they are. In this figure from Timothy A. Livengood's proposal, ratios of colors (indicated by their wavelengths) sort the planets into distinct groups using color information. The Earth, with its water and life, is distinct from the other planets in the solar system.

While this blog is about planetary studies, exciting, potentially
ground breaking astronomical missions also were proposed – it is not a given
that a mission using the second telescope would have a planetary focus. But if I were to answer my own question about
what I would use a Hubble-class telescope for, my first choice would be to
study planets around nearby stars. This
would be ground breaking research. If
the budget and spacecraft capabilities permitted, I’d then add instruments for studying
solar system objects and dedicate large blocks of time to solar system studies. (These instruments also could be similar –
but with more modern technologies – to those currently on Hubble and therefore
would also be useful for continuing Hubble astronomy studies.)

The recently completed conference was intended to look at a wide range
of concepts. NASA intends to select six
for more in-depth studies. It is likely
to have plenty of time to do so. Given
current budgets, the second telescope (assuming the first is used for the WFIRST
mission) seems unlikely to fly until the mid-2020s, if ever.

We've had a 'wake-up call' on just how unprepared we are to spot objects coming our way, and folks want to send *TWO* telescopes that could be put in orbit to spot more objects off to do something else?

For Apophys, I'm not a big fan of a hydrogen explosion "air burst", because of the unpredictability of the physics of the meteor. But if this contingency is selected, it would make sense to have a whole bunch of observatories pre-launched, to view the hydrogen bomb "radar signature" across the solar system and beyond. It would allow us to image nearly everything, hitting all the birds with one nuke. Where resources aren't competing with the primary mission.

About Me

You can contact me at futureplanets1@gmail.com with any questions or comments.
I have followed planetary exploration since I opened my newspaper in 1976 and saw the first photo from the surface of Mars. The challenges of conceiving and designing planetary missions has always fascinated me. I don't have any formal tie to NASA or planetary exploration (although I use data from NASA's Earth science missions in my professional work as an ecologist).
Corrections and additions always welcome.